mmc: core: Fixup Oops for SDIO shutdown

Commit "mmc: core: Handle card shutdown from mmc_bus" introduced an
Oops in the shutdown sequence for SDIO.

The drv pointer, does not exist for SDIO since the probing of the SDIO
card from the mmc_bus perspective is expected to fail by returning
-ENODEV.

This patch adds the proper check for the pointer before calling it.

Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Reported-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>